Slurry flow control



Nov. 12, 1968 FINES SETTLER G. c. BLYTAS ETAL 3,410,795

SLURRY FLOW CONTROL Filed March 1-7, 1967 g E E E .J 3 w LU g In 8 3 mcol 0 2 LL] 6 Q J 8 5 05 J E 5 Z (I) INVENTORS= GEORGE c. BLYTAS E.ROBERT FREITAS THEIR ATTORNEY United States Patent F 3,410,795 SLURRYFLOW CONTROL George C. Blytas, Albany, and Ernest Robert Freitas, SanLeandro, Calif., assignors to Shell Oil Company, New York, N.Y., acorporation of Delaware Continuation-impart of application Ser. No.373,451, June 8, 1964. This application Mar. 17, 1967, Ser. No. 638,166

9 Claims. (Cl. 210-33) ABSTRACT OF THE DISCLOSURE A method forregulating the transfer flow rate of a slurry in which the flow iscontrolled by introduction of the slurry into a circulating system whereit is diluted with a recycle fluid, throttled and passed to a separationzone where the dilute slurry is separated into recycle fluid andreconcentrated slurry which is discharged from the system at the desiredflow rate thus allowing flow control without the plugging and attritionassociated with conventional throttling means. The separation zone maybe used to separate solid fines from the slurry in which case solidfines are removed from the recycle fluid before the latter isrecirculated.

Cross reference to related applications Background of the invention Thisinvention relates to a method of process control in a solid-fluidsystem. In particular, this invention relates to control of flow andremoval of fines from a slurry system.

It is well known to employ slurries of particulate solids in fluid,especially liquid carriers, in various processes. For example, Broughtonet al., U.S. 2,957,927, patented Oct. 25, 1960, teaches the separationof normal hydrocarbons from hydrocarbon mixtures with a slurry of asolid silicate suspended in a liquid organic amine. The removal of anadsorbable gas from a gas mixture with a charcoal-in-water slurry isdescribed in Maslan, US. 2,823,- 765, patented Feb. 18, 1963. Marple etal., US. 2,768,942, patented Oct. 30, 1956, teaches an adsorptivedistillation process employing a countercurrent slurry adsorbent.Slurries of solid catalysts, e.g., for conversion processes,

mineral ores, crystals, pulps, abrasives, pigments, etc. are r alsoused.

In some processes, for example, crystallization from a mother liquor, itis very diflicult to avoid processing a slurry. In other processes,especially those using slurries for physical transportation of solids,e.g., ore processing, dilute concentrations of solids are employed toinsure carrying free of plugging and hold-up. Although sorption andcatalytic processes involving slurried solids overcome certaindisadvantages inherent in fixed-bed processes, such as non-continuousoperation and variation of product composition, they have not achievedwidespread commercial use generally because of materials handlingproblems, such as flow control, solids attrition, etc. It is readilyapparent that a slurry containing, e.g., -40% volume solids (i.e.,percent of total slurry volume occupied by solids) cannot be meteredthrough standard control valves without losses of efliciency andaccuracy, plugging, and considerable wearing, chipping and breaking ofsolid particles.

conventionally the flow of fluids are regulated by restrictions in theconduit containing the flowing medium, the most common restrictionsbeing of course, control 3,410,795 Patented Nov. 12, 1968 valves.However, when dealing with slurry systems containing substantial amountsof solids, conventional means are ill suited to the job. The physical.obstructions in the valve, necessary to throttle the fiow,result inpractical problems which are diflicult to overcome. Not only is thevalve rapidly damaged by erosion, but the solids are often attrited toan unacceptable degree. Perhaps most serious however, is the problem ofplugging. Most flow control devices introduce into the system physicalobstructions whichallow solids to accumulate and pack. As the solidsaccumulate and pack the conduit gradually becomes plugged. If theprocess of plugging is gradual, it is often overcome by increasing thedriving pressure or reducing the mechanical restriction, as by openingthe control valve. The system eventually becomes completely plugged andmust be cleaned out or, as commonly happens, the plug suddenly breaksloose causing a surge in flow. The latter situation can in some caseshave more serious consequences than plugging per se and prevents stablecontrol of a circulating slurry system.

Thus, the problems associated with plugging the flow control device haveperhaps been a major deterrent to extensive commercial use ofcirculating slurry processes.

Particle attrition incident to prior methods of flow control can resultin the accumulation of fines, which are particles considerably smallerthan most other particles in the slurry and are desirably removed fromthe system. Although some fines production is attributable to particlefracture resulating from strain induced in the solids by, e.g.,heating-cooling or adsorption-desorption cycles, a major amount of finesproduction is caused by wearing and chipping of particles due to impactwith other particles, pipe walls and equipment. Fines productionincreases as more concentrated slurries are used because of increasedinter-particle contact. Breaking and chipping of particles is especiallyprevalent when a concentrated slurry passes through narrow apertures atrelatively high velocities, such as through a manual or control valve.

Fines are generally undesirable constituents of most slurries forvarious reasons. In some processes wherein adsorbents or catalysts areused in slurries, a critical solid surface-to-volume ratio might beupset by the accumulation of fines; in other systems fines build up ashard deposits and cause plugging. Maintenance of a low amount of finesin the system is especially important in countercurrent processes (i.e.,where the solid flows countercurrent to the net liquid flow), since itis essential to maintain a certain minimum settling rate for the solidparticles.

Summary of the invention It has now been discovered that an effectivemethod of flow control in slurry systems comprises in sequence 1)diluting the slurry with recycle fluid, (2) regulating the flow of thediluted slurry, (3) separating the slurry into reconcentrated slurry andrecycle fluid, (4) returning the recycle fluid for use in diluting moreslurry in step (1). In one advantageous embodiment fines may be removedinto the recycle fluid in step (23) and the recycle fluid separated fromthe fines before recycle. This combination of process steps isparticularly advantageous for several reasons; As indicated above, evencoarse flow control of slurries containing more than about 3-5% volumesolids may be very diflicult, even with special equipment such aselastic diaphragm valves, because of frequent plugging of the valves.Although this problem could be eliminated through the use of a moredilute slurry throughout the process, higher costs and lower processefliciencies would result. The present invention obviates plugging bythe use of a recycle stream of carrier fluid which eflectively dilutesthe slurry for the purpose of flow control, but returns a concentratedslurry to the process. Substantial dilution of the slurry together withflow control and I 3 throttling provides sufficient turbulence to thesystem to prevent plug formation. The excess liquid together with thesubstantial increase in flow produced by inclusion of the diluentcoupled with the resulting low concentration of solids prevents pluggingof restrictions in the system and allows smooth and stable flow control.Moreover, variation in diluent flow provides a means for slurry flowcontrol independent of' the throttling valve or device.

By flow control as used herein is meant the rate of transfer of slurryfrom one point to another, that is, the regulation of transfer ratebetween, for example, one process unit section and another. The controlis independent of the conditions of slurry concentration and pressure inthe processing-units. While the existing pressure differential betweenthe points of transfer and the physical nature of the system set themaximum flow, regulation of flow below maximum is achieved by means ofthe circulating system of the invention. Thus, the circulating recyclefluid serves two functions: dilution of the slurry and as a means ofregulating pressure drop between points of transfer. Variation inrecycle fluid circulation rate provides a means of producing flowregulating pressure drop independent of throttling. For example, in asystem of fixed throttling restriction, the transfer of process slurrythrough the circulating control system can be controlled by variation inrecycle flow. If it is desired to slow down the transfer of slurry,recycle rate is increased. The increased total flow results in higherpressure drop in the circuit and net slurry flow is reduced. Converselyif greater slurry flow is desired, the recycle rate is reduced. Theinvention allows regulation of slurry flow independent of the pressureand slurry concentration of the points of transfer. Thus, while thecontrol system involves dilution, recirculation and reconcentration, itdoes not require changes in the slurry or operating conditions of thepoints of transfer. In this respect it is like any other flow controldevice. Unlike other methods of slurry flow'control it minimizes theproblems of attrition and plugging, and control may be varied by varyingrecycle rate with varying the throttling restriction.

Controlling the flow of diluted slurry provides the further advantage ofreducing the amount of fines produced by particle attrition, incident topassing concentrated slurries through small, irregular passageways ofstandard control valve or devices. Although the process of the inventionis particularly applicable to slurries of solids in liquid carriers, itis also applicable to solid-gas systems. Therefore, the term slurry isintended to mean solid-fluid mixtures, including both solid-liquid andsolidgas mixtures. For convenience, the invention will be discussed interms of solid-liquid slurries.

In one embodiment the present combination of process steps provides avery convenient method for removal of fines from systems where finesremoval is desired. The

solids-disengaging, or slurry-reconeentrating step is easily operated insuch a manner as to allow at least a portion of the fines to pass outwith recycle liquid, from which they or a portion of them are removed bysettling or filtering. Fines as discussed herein are particles which areconsiderably smaller than most other particles in the slurry and whichare desirably removed from the system. Generally, fines have a settlingvelocity in the liquid carrier of less than cm./sec., preferably lessthan 3 cm./sec., although this figure may vary widely with the size ofparticles and type of process involved. For many common applications,the fines are solid particles which pass through a 65 mesh (210 Tyler)sieve. The present process allows removal of these fines without anadditional separation of fines from larger particles, said separationbeing in the present case incidental to the reconcentration step.

The fines which are removed from the recycle stream prior to theaddition of the recycle stream to the slurry are produced by attrition,which is herein defined to be the production of smaller from largerparticles, e.g., by

wear, chipping or breaking. Attrition is experienced in all slurryprocesses, and is generally attributable either to particle impact withother particles, walls and equipment, or to particle fracture resultingfrom strain induced in the solids by, eg., heating and cooling, oradsorption and desorption. Fines are desirably removed from slurries forvarious reasons as discussed above. 1

Dilution of the slurry prior to flow regulation may be done ina'separate vessel but is preferably carried out directly in a pipeline.In general; slurries useful in various processes contain from about 3%to about 55%,'preferably 7% to about 40% by volume solids. In accordwith the invention, these slurries are diluted to from l 20%, preferably215% by volume solids, depending on the total flow of solids desired,and type of particles and process concerned. It is the appreciabledilution which allows stable and eflicient flow control.

As mentioned above, the invention is applicable to any process employinga solid-fluid slurry. The solid material may comprise chemicallyreactive or inert materials, such as sorbents, catalysts, abrasives,crystals, ores and pigments.'Many solids, such as mined ores, wood,sand; shale, and coal are slurried to provide convenient methods oftransporting the solid material. Other solids often found in slurriesare chemicals supported on inert carriers, solid clathrates, chelates,and other solid chemical complexes. Preferred solids are catalysts andadsorbents; adsorbents include, for example, silica gel, metalloaluminumsilicates (molecular sieves), activated carbon, charcoal, and natural orsynthetic zeolites. Catalysts include aluminum chloride, supported metalcatalysts, etc. Particle diameters are generally at least 5 microns,preferably 10 microns to mm., especially microns to 10 mm. Liquidcarriers may be water, organic amines, aromatic and/ or aliphatichydrocarbons, molten salts, solvents, etc.

Flow control may generally be effected in a throttling zone, comprisingany device for restricting the flow in the line, Manual or automaticcontrol valves, orifices, or coils are suitable. Although it ispossible, when using a diluted slurry, to effect complete control of theflow by the throttling step, it is preferred to effect only coarse(i.e., manual) control of the flow in the throttling step, and to effectclose control of the slurry flow by adjusting the flow rate of therecycle stream. At a given valve opening, the pressure drop issubstantially controlled by the total flow of slurry plus recycleliquid. Thus, at a given pressure drop, the slurry rate is readilycontrolled by regulating the rate of the recycle stream, since the totalrate remains substantially constant.

While the use of a throttling valve is desirable in many cases to obtainmore flexibility of control, there is some advantage for use of a fixedthrottling zone such as a coil. It is a well founded principle that thepressure drop in a coil depends upon the dimensions, i.e., its lengthand cross sectional area, the radius of curvature, and the nature of thefluid flowing through the coil. Conversely, the flow depends upon thepressure drop,

When the radius of curvature of coil is about 5 or more times that ofthe conduit diameter itself, the pressure drop effected by a givenlength of coiled conduit is about twice that which would result from thesame length of uncoiled conduit. When the radius of curvature of theconduit is less than about 5 times the conduit diameter,

" the pressure drop increases sharply. For a given coil the pressuredrop across the coil can be regulated by the amount of flow through thecoil. Therefore, the amount of diluent circulated can be used as adirect means for controlling the pressure drop and consequently thetransfer of concentrated slurry drawn into and taken out of thecirculating flow control system.

Slurry reconcentration is continuously carried out, sub sequent tothrottling, in a solids-disengaging zone. This zone concentrates theslurry usually to essentially the same concentration as before dilution,and yields a slurry stream and a stream of liquid (recycle liquid) whichmay contain fines, including fines which may have resulted from theturbulence caused by the throttling device. It is desirable to removefines at the same rate that they are produced in the overall slurrysystem; fines maybe removed at'more than one location in the system, ofcourse. Any known solids-concentration device is acceptable as. thesolids-disengaging zone, for instance, centrifuges, filters, cyclones orelutriators. Elutriators are devices which permit slow upward flow ofliquid, heavier solids settling to the bottom and being drawn off as aconcentrated slurry. Separated liquid and entrained fines are drawnoffat the top of the elutriator. A simple form of an elutriator isexemplified by a vertical pipe with an outlet near-the bottom forwithdrawing concentrated slurry. Dilute slurry enters the bottom of thepipe; liquid and fines flow from the top. The rate of upward flow ofliquid in the elutriator is determined by the settling rate of the finesdesirably removed; the flow rate must be greater than the settlingvelocity of the largest particle to be carried overhead. Althoughseveral devices are available for slurry concentra tion, it is essentialto carry out this step continuously to maintain flow control.

' Recycle liquid with entrained fines passes from the slurryconcentration step to a fines removal step. The removal of very smallparticles may be carried out inany conventional and well establishedmanner, for instance, by settling, centrifugation, or filtration. Afterremoval of at least part of the fines, the recycle liquid is reused fordilution of incoming slurry. Where fines removal is not carried out, thediluent may be stored in a surge or hold-up vessel before beingrecirculated as diluent.

Description of drawing and specific embodiment A specific example of theflow-control method of the invention with fines removal is illustratedin the drawing. Referring to the drawing, slurry (from a main process)consisting of about 25% volume solids passes from slurry inlet 1 intoline 2, where it is diluted with recycle liquid to form a diluted slurryof about 6% volume solids. Diluted slurry passes through line 2 andmanual throttling valve 3, which is used for a coarse control of theflow; i.e., to set the total flow of slurry plus recycle liquid. Acontrolled amount of diluted slurry passes through line 4 and intoelutriator 6, wherein the slurry is continuously reconcentrated to about25% volume solids, essentially the same solids concentration as theslurry entering line 1. Liquid and fines if any pass upward at the rateof about 0.1 ft./sec.; concentrated slurry is withdrawn near the bottomof the elutriator and is passed to another process step. Recycle liquidand entrained fines pass from the elutriator through line 7 to a finessettler, which is a large vessel with a means for removing settledparticles near the bottom. Recycle liquid is withdrawn near the top ofthe fines settler and is passed through line 9 and pump 10 and isreturned to line 2 where it again dilutes incoming slurry. The flow ofrecycle liquid is adjusted by automatically controlling with controlvalve 12 the amount of liquid flowing through pump bypass 11; e.g., whenvalve 12 is closed, greater amounts of recycle liquid flow into line 2,thereby reducing slurry flow. Where fines removal is not required ordesired, a settler may be substituted for the elutriator forsolid/liquid separation. Operating fiow rates would be more flexible forthe slurry settler and the surge vessel.

As mentioned above, the combination fines-removal and flow controlscheme of the invention may be broadly applied to any process wherein asolid-fluid slurry containing more than about 3% volume solids isemployed. The invention is particularly applicable to sorption processeswherein the solid acts as a selective sorbent for one or more compoundsdesirably removed or separated from a liquid or gas mixture, and wherethe solid passes countercurrent to the net flow of liquid or gas. Thecontrol process of the invention is especially advantageous when appliedin a process using a rotating-disc contactor, such as for removal ofaromatics from a feed to a Friedel-Crafts isomerization process. It iswell known that benzene has a deleterious eflect on the activity ofFriedel-Crafts catalysts, such as AlCl and AlBr which are used in thelow temperature isomerization of normal butane, pentanes and hexanes. Toprevent sludge formation and catalyst deactivation, the tolerance ofbenzene in a reactor of this type is in the order of 0.1% byweightbenzene. Although the separation of aromatics can ,be accomplishedby such methods as solvent extraction, extractive distillation, and lowpressure hydrogenation with, e.g., noble metal catalysts, suchoperations are costly. Fractionation of the feed to remove benzene isimpractical, as removal of benzene to the required level by this methodwouldresult in substantial losses of n-hexane and methyl pentanesbecause of the small differences in boiling points as well as azeotropeformation. Dunn et al. have discovered. that benzene may be selectivelyremoved from isomerization feedstocks by contacting the feedswith asolid adsorbent such as silica gel in a rotating-disc contactor (RDC).This process is described in Belgian Patent 627,719, Sept. 18, 1963.

In this embodiment of the invention, rotating-disc contacting zones asdescribed in Reman, U.S. 2,601,674, June 24, 1952, are employed incombination with a solid adsorbent slurry to selectively adsorb benzenefrom the isomerization feed. While the invention of Reman deals withliquid-liquid extraction system the device of the invention may beadapted for solid/liquid contacting. The present invention isparticularly applicable to this process, since rotating-disc contactorperformance is very sensitive to the presence of fines and change inflow. A slurry of silica gel in an essentially aromatic-free paraflinhaving 4-6 carbon atoms per molecule is maintained in a slurry storagezone, or sump, from which slurry is removed for use in thedearomatization process. Slurry concentrations may be adjusted, andmakeup gel may be added to this zone. Slurry of desired concentrationpasses from the storage zone and, in accord with the invention, isdiluted with recycle parafiin, passed through a throttling valve, andreconcentrated inan elutriator. Recycle liquid and entrained gel fines,which fines have been produced by wearing, chipping and cracking of thelarger gel particles, are removed from the top of the elutriator and arepassed to a fines settler, wherein the small gel particles are allowedto settle and are removed. The recycle liquid then passes through apump, and a regulated amount of liquid is recombined with incomingslurry from the slurry storage tank. Reconcentrated slurry from theelutriator is then passed to a first rotating-disc contacting zone.Thus, the rate at which slurry is transferred from storage to thecontact zone is controlled by the circulation system of the invention.

In the first rotating-disc zone, the silica gel-parafiin slurry isintroduced near the top of the contacting zone. The gel falls throughthe upper or adsorption section of the zone and selectively adsorbsbenzene from the upfiowing feed which was introduced into a lowerportion of the adsorption section. It is essential that the slurrycontain an importantly low amount of fines, which would be sweptoverhead or would not maintain an adequate settling velocity for highprocess efiiciency. The benzene-free feed passes overhead from the firstrotating-disc contacting zone, the silica gel having adsorbed benzene onits surface. The slurry is then washed with a small stream ofisomerizate-rich material in an exchange-Wash section of the contactor,thereby minimizing bypassing of feed paraffins around the isomerizationreactor. This exchange wash is described in detail in Belgian Patent629,719 mentioned above.

The gel slurry leaves the first rotating-disc zone and is heated andpassed into the upper section of a second rotating-disc contacting zonewhere an upfiow of hot benzene-free isomerizate (product from thesubsequent isomerization reaction) strips the benzene from the gel atabout 275 F. The isomerizate and benzene are passed out ofthe desorptionsection and may be advantageously used in gasoline blending. In a lowersection of the'second rotatingadisc zone, adsorbed isomerizate isdisplaced from the gel surface by a small stream of benzene-free feedparaffin obtained from the top of the first RDC.

Concentrated slurry is returned to the storage zone described above, andmay optionally be passed through the flow control and fines removalsteps of the invention before entering the storage zone.

We claim as our invention:

1. A method of controlling the flow rate of a slurry containing at least3% by volume particulate solids comprising the following steps insequence:

(1) diluting the slurry with a recycle fluid,

(2) throttling the diluted slurry,

(3) separating the diluted slurry into a more concentrated slurry and arecycle fluid,

(4) returning the recycle fluid to step (1), and

(5) adjusting the throttling restriction of dilute slurry and thecirculation flow rate of recycle fluid to obtain the desired transferflow rate of process slurry.

2. The method of claim 1 wherein the recycle fluid of step (3) containsentrained solid fines and the fines are separated from the recycle fluidbefore returning the recycle fluid to step (1).

3. The method of claim 1 wherein the particulate solids comprise acatalyst.

4. The method of claim 1 wherein the particulate solids comprise anadsorbent.

5. The method of claim 1 wherein the process slurry contains about 3 toabout 55% by volume particulate solids and the dilution with recyclefluid in step (1) is effected to obtain a concentration of from 1% to20% volume particulate solids.

6. The method of claim 1 wherein the concentrated slurry of step (3) isessentially the same concentration as the undiluted process slurry.

7. A method of controlling the flow rate of transfer of a process slurrycontaining from about 7 to about 40% by volume particulate solidscomprising the following segmented steps:

(1) diluting the slurry with a recycle fluid to a concentration of from1% to 20% volume particulate solid,

(2) throttling the diluted slurry,

(3) separating the diluted slurry into a concentrated slurry ofessentially the same concentration of solids as the undiluted processslurry and a recycle fluid,

(4) separating solid fines from the recycle fluid,

(5) returning the recycle fluid from which solid fines have been removedto step (1), and

(6) adjusting the throttling restriction of dilute slurry and thecirculation flow rate of' r'ecycle fluid to obtain the desiredtransfer'flow rate of process slurry.

8. In a sorption process for removing at least 'one component from afluid which comprises passing in a contact zone a particulate solidsorbent in flow countercurrent to the net fluid flow, at least onecomponent be ing removed on the sorbent, the sorbent particles beingsupplied to and removed from the contact zone as a slurry, theimprovement therein which comprises controlling the flow of slurry by amethod characterized by sequentially:

(1) diluting the slurry with a recycle fluid,

(2) throttling the diluted slurry,

(3) separating the diluted slurry into a more concentrated slurry and arecycle fluid,

(4) returning the recycle fluid to step (1), and- (5) adjusting thethrottling restriction of dilute slurry and the circulation flow rate ofrecycle fluid to obtain the desired transfer flow rate of processslurry.

9. The process improvement of claim 8 wherein flow of slurry iscontrolled by sequentially:

(1) diluting the slurry with a'recycle fluid to a concentration of from1% to 15% volume particulate solid,

(2) throttling the diluted slurry,

(3) separating the diluted slurry into a concentrated slurry ofessentially the same concentration of solids as the undiluted processslurry and a recycle fluid,

(4) separating solid fines from the recycle fluid,

(5) returning the recycle fluid from which solid fines have been removedto step (1), and

(6) adjusting the throttling restriction of dilute slurry and thecirculation flow rate of recycle fluid to obtain the desired transferflow rate of process slurry.

References Cited UNITED STATES PATENTS 2,654,479 10/1953 Driessen 210195X 2,756,685 7/1956 Hoogendonk 302-14 X 2,793,082 5/1957 Gardner 302-142,823,763 2/1958 Maslan 55-22 2,886,210 5/1959 Cooper et al. 302-14 X2,915,336 12/1959 Vaell 30214 3,272,335 9/1966 Nettel 210-73 X 3,325,0116/1967 Keller 21096 REUBEN FRIEDMAN, Primary Examiner.

C. DITLOW, Assistant Examiner.

